β-Amanitin is a cyclic peptide found in several species of the Amanita genus, as well as other poisonous fungi. It inhibits RNA polymerase II and III but not RNA polymerase I or bacterial RNA polymerase, thus preventing mammalian protein synthesis. The biological activity of this toxin is similar to that of α-amanitin but differs structurally by the presence of a carboxyl group, which is useful for coupling reactions.
